1. Field of the Invention
This invention relates to a load interrupter switch for voltages above 1 kV. More specifically, this invention relates to load interrupter switch having a vacuum interrupter chamber whose contacts are closed or opened using a switching mechanism, wherein the vacuum interrupter chamber has a housing with metallic end plates which encloses the switching contacts situated in the vacuum, and a cylindrical housing middle part made from an electrically insulating material, of which the housing is surrounded by a dielectric medium.
2. Description of Related Art
Load interrupter switches are commonly used as switch-disconnectors in railway operations. In such a case, in the closed position, the vacuum interrupter chamber together with the switching mechanism accommodated in an insulating housing, is electrically connected in parallel with the traction circuit designed for the full nominal equipment current. During disconnection, the main contacts are opened when de-energized and in the process transmits the current into the vacuum interrupter chamber and auxiliary switching point, which has an actuating fork. As soon as the main contacts have moved apart far enough from one another, the vacuum interrupter chamber is quickly actuated via a tilting mechanism. The breaking arc occurring in the interior of the interrupter chamber is extinguished at the first current zero without appearing externally.
However, such vacuum interrupters or interrupter chambers are relatively large and have high production costs. Consequently, because of the size and cost of vacuum interrupter chambers, vacuum interrupter chambers using a lower voltage series than that for which the switches have been designed are commonly used. However, it is possible to reduce both the dimensions and the production costs of conventional vacuum interrupter chambers.
However, the reduction in overall size, also attended by reduces the spacing of the metallic end plates of the housing of the vacuum interrupter chamber. In this case the external insulation, which is stressed during and after the disconnection, is insufficient in size to prevent air from flowing from or into the surrounding environment.
In order to solve this problem, the vacuum interrupter chambers are arranged in a medium of higher dielectric strength. It is possible in this case to apply insulating oil, such as mineral oil or silicone oil, various esters or an insulating gas such as, sulphurhexafluoride (SF.sub.6). These mediums displace the air in the surroundings of the vacuum interrupter chambers and, since they have a high dielectric strength, an external flashover is prevented.
However, the use of such mediums may cause objectionable environmental hazards. Since such load interrupter switches are used for many years, the probability of leaks due to aging of the components of the load interrupter switches is greatly increased along with the probability that such of the mediums may escape into the environment.
A further disadvantage of such mediums is that they require continuous monitoring. When using insulating oil, periodically checking the oil level is necessary. Since such load interrupter switches are installed on high masts in most cases, a corresponding outlay is required. The situation is similar when using insulating gas, wherein the pressure must be checked periodically.
Encapsulation using epoxy resin, may also be used to improve the external insulation of the vacuum interrupter chambers by. However, as the vacuum interrupter chamber ages, an air gap may occur which may lead to an external flashover in the region between the epoxy resin casing and the outer housing. Such aging may cause stress cracks due to after-shrinkage of the cast resin jacket, and embrittlement due to loss of effectiveness by flexibilizers which were used during encapsulation. As a consequence of differential material expansion during frequent alternating stress between hot and cold, the formation of gaps by detachment of the resin jacket from the outer housing of the vacuum interrupter chamber may occur. This risk cannot be entirely removed or ignored. The fact that such an encapsulated vacuum interrupter chamber can be accessed when dismounting only by destroying the enclosure is a further disadvantage.
The embodiments disclosed above are complicated and may be used only on a conditionally basis, and are frequently rejected because of possibility of endangering the environment.
The French patent FR-2 698 481 A1 discloses a load interrupter switch having a vacuum interrupter chamber, an electrically insulating body made from silicone being arranged between the housing of the vacuum interrupter chamber and an outer housing. The silicone body is tubular and has elastically deformable ribs either on the outside or on the inside. It is made so that the simultaneously makes intimate contact with the outer surface of the interrupter chamber housing and the inner surface of the outer housing. The aim here is to achieve an absence of a gap in order to avoid an electric flashover. In addition, it is possible during mounting to introduce an insulating grease in the region between the interrupter chamber housing and the inside of the silicone body, while the ribs on the outside are compressed at least slightly in order to produce a seal.
German Utility Model G 93 14 754 U1 has disclosed a vacuum interrupter having an encapsulation resistant to internal pressure. The encapsulation of this vacuum interrupter comprises an inner coating made from a hard foam plastic, and an outer burst-proof jacket. The inner coating, preferably consisting of a polyurethane foam, which is uniformly porous to permit the best possible thermal insulation. The inner coating will prevent the temperature from rising to a sufficient level to ignite the surrounding gas. The burst-proofjacket is constructed as a wound body, and comprises threads or strips which are impregnated with a cured plastic. It is constructed bearing tightly against the foam coating and dimensioned such that it can absorb the bursting force which occurs in the event of a fault inside the vacuum interrupter:
However, the sheathing of the interrupter includes a permanently foamed plastic material whose properties can be impaired by aging. In particular, embrittlement or detachment of the foam coating from the outer housing of the interrupter can occur. In addition, this encapsulated vacuum interrupter can be dismounted only given destruction of the enclosure.
Therefore, a load interrupter switch that is capable of use over a long period of time, which does not require monitoring and may be dismounted is needed.